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A Quantum Delayed-‐Choice (QDC) Experiment
John Archibald Wheeler 1983
A Quantum Delayed-‐Choice (QDC) Experiment
A Quantum Delayed-‐Choice (QDC) Experiment
o Quantum systems exhibit par$cle-‐like or wave-‐like behaviour depending on the experimental apparatus that they are confronted with! – Delayed Choice experiment (DC)
o Photon is forced to choose a behaviour before the observer decides what to measure! (DC)
o Quantum delayed-‐choice experiment (QDC) – parCcle and wave behaviour invesCgated
simultaneously!
o VerificaCon of the quantum nature of the photon’s behaviour is via nonlocality with QDC, replacing the delayed choice of the observer in DC.
A QDC Experiment – OVERVIEW
CLAIM: “we observed strong nonlocal correla$ons, which show the photon must SIMULTANEOUSLY behave
both as a par$cle and a wave”
**KEY DEVELOPMENT: NO LONGER NECESSARY TO CLOSE/OPEN INTERFEROMETER!**
(therefore removing the role of the observer/device!) o QDC scheme based on Bell’s inequality (21) – can test the most general classical model
-‐ device-‐independent
“we cer$fy the quantum nature of the photon’s behaviour by observing the viola$on of a Bell inequality. This demonstrates in a device-‐independent way (without making assump$ons about the
func$oning of the devices – that no local hidden variable model can reproduce the quantum predic$ons.”
A QDC Experiment – OVERVIEW
o Wave/par$cle duality -‐-‐-‐-‐àDESCARTES 1630 (waves) NEWTON 1670s (parCcles) vs HUYGENS 1678 (waves)
-‐-‐-‐-‐à YOUNG 1803 (waves) – Double Slit -‐-‐-‐-‐à MAXWELL 1865 (waves), verified by HERTZ 1887
o NoCon of COMPLEMENTARITY (Copenhagen interpretaCon) -‐-‐-‐-‐à Appeal to hidden-‐variable models o Wheelers DELAYED CHOICE (DC) gedankenexperiment 1980s
A QDC Experiment – LANDMARKS
o Experimental implementaCons (1987 – 2000)
A QDC Experiment – LANDMARKS
A DC Experiment – WHEELER
John Archibald Wheeler 1983
A DC Experiment – WHEELER
A DC Experiment – WHEELER
Squint to see a ‘complementary’ image!!! J A DC Experiment – WHEELER
Squint to see a ‘complementary’ image!!! J A DC Experiment – WHEELER
o Describes dialog between EINSTEIN and BOHR between EINSTEIN’S arrival in Princeton in 1933 and his death in 1955.
o EINSTEIN is adament that an observer dependence “IS INCOMPATIBLE WITH ANY REASONABLE IDEA OF REALITY”
o BOHR’S reply “YOUR CONCEPT OF REALITY IS TOO LIMITED’
o EINSTEIN could not reconcile himself to the fact that the DOUBLE-‐SLIT experiment could be made to demonstrate WAVE-‐LIKE or PARTICLE-‐LIKE behaviour.
o He declared it to be “CLEARLY OBVIOUS THAT QUANTUM THEORY IS INCONSISTENT”
o BOHR emphasised that there in fact was NO INCONSISTENCY
o We are dealing with two different experiments, BUT IT IS IMPOSSIBLE TO DO BOTH EXPERIMENTS SIMULTANEOUSLY (unCl the QDC!!!)
o He argued one can observe one feature of nature, or its complementary feature. BUT NOT BOTH SIMULTANEOUSLY (unCl the QDC!!)
A DC Experiment – WHEELER
A DC Experiment – BACKGROUND
A DC Experiment – BACKGROUND
A DC Experiment – BACKGROUND
o Decision to add / remove half-‐silvered mirror (BS2) is made at the last picosecond, aher the photon has already travelled through (most) of the system
o “WE HAVE A STRANGE INVERSION OF THE NORMAL ORDER OF TIME” (unCl QDC!!) o “WE NOW…HAVE AN UNAVOIDABLE EFFECT ON WHAT WE HAVE A RIGHT TO SAY ABOUT
THE ALREADY PAST HISTORY OF THAT PHOTON”
o Argument has since been refined with the DC experiment
A DC Experiment – WHEELER
A DC Experiment – WHEELER
o DC AT THE COSMOLOGICAL SCALE!
o Distance of travel in a Lab split-‐beam experiment ~30m o Time of travel ~ 0.1 microseconds
o Distance COULD be billions of light years
o Consequently an observing device, configured to respond a specific way has an “IRRETRIEVABLE CONSEQUENCE FOR WHAT ONE HAS THE RIGHT TO SAY ABOUT A PHOTON THAT WAS PRODUCED LONG BEFORE THERE WAS ANY LIFE IN THE UNIVERSE”
o Two astronomical objects (0957 + 561 A,B)
(Were once considered to be two disCnct quasi-‐stellar objects (or ‘quasars’) as they are seperated by 6 seconds of arc. Now considered to be two disCnct images of one quasar An intervening galaxy has been found 1/4 of way from us to the quasar
(0957 + 561 A,B) A DC Experiment – WHEELER
A DC Experiment – WHEELER
A DC Experiment – Experimental RealisaCon
A DC Experiment – Experimental RealisaCon
A DC Experiment – Experimental RealisaCon
A DC Experiment – Experimental RealisaCon
A DC Experiment – Experimental RealisaCon
o University of Rochester – Emission of photon pair from two-‐down conversion crystals
o One photon passes MZI, second photon determines whether first shows interference
o THEREFORE view that act of determining parCcles path disturbs interference effects is UNTENABLE!
o KEY FACTOR = whether path informaCon is available or not (experiments where path info carried by second parCcle is destroyed aher parCcle registered – see interference!)
A DC Experiment – Experimental RealisaCon
o QDC says otherwise!!
A DC Experiment – Experimental RealisaCon
A DC Experiment – Experimental RealisaCon
A DC Experiment – Experimental RealisaCon
A DC Experiment – Experimental RealisaCon
o QDC says otherwise!! (see next slide)
A QDC Experiment – Proposal
A QDC Experiment – Proposal
A QDC Experiment – Proposal
A Quantum Delayed-‐Choice (QDC) Experiment
o Quantum mechanics most consistent and accurate physical theory we have...
o HOWEVER, makes counter-‐intuiCve predicCons
o NoCon of WAVE-‐PARTICLE DUALITY introduced A quantum sytem may behave as a parCcle or wave depending upon the experiment
o COPENHAGEN INTERPRETATION – parCcle / wave behaviour are never observed simultaneously
o To reconcile quantum predicCons and common sense it has been suggested that quantum parCcles may know the experiment in advance, via a HIDDEN VARIABLE
o Challenged by J. A Wheeler in DELAYED CHOICE gedankenexperiment
A Quantum Delayed-‐Choice (QDC) Experiment
o Quantum parCcle sent towards MZI o RelaCve phase between the two MZI arms adjusted so parCcle emerges at D’ with certainty
(WAVE-‐LIKE BEHAVIOUR) o BS2 can be removed arbitrarily – meaning a WHICH PATH measurement is made. o The photon is detected in each mode with an equal probability of 1/2
(PARTICLE-‐LIKE BEHAVIOUR)
o BS2 can be removed/inserted arbitrarily aher the parCcle has entered the MZI (aher BS1) o ParCcle could not have known in advance (via hidden variables) which kind of experiment it would
be confronted with.
A Quantum Delayed-‐Choice (QDC) Experiment
CONCEPTUALLY DIFFERENT STARTING POINT: o Recent theoreCcal proposal of QDC using a quantum-‐controlled BS (C(H)) which can be in a
superposiCon of PRESENT and ABSENT o meaning the MZI can be simultaneously CLOSED and OPEN – tesCng WAVE and PARTICLE nature of
the photon simultaneously!
o Used a reconfigurable integrated quantum photonic circuit to implement a MZI with a quantum BS o Observed CONTINUOUS MORPHING between WAVE and PARTICLE behaviour! o (although this can be reproduced by a simple classical model – weakness in proposal and recent
NMR implementaCons)
A Quantum Delayed-‐Choice (QDC) Experiment
o Observed CONTINUOUS MORPHING between WAVE and PARTICLE behaviour!
o Therefore experimentally demonstrated QDC based on BELL INEQUALITY – allowed to test the most general classical model.
o ACHIEVED STRONG BELL INEQUALITY VIOLATIONS – the delayed choice of closing the MZI or not is no longer necesssary – instead we cerCfy the quantum nature of the photon’s behaviour by observing the violaCon of a Bell ineuality
o Demonstrates in a device-‐independent way (without making assumpCons about the funcConing of the devices) – that NO LOCAL HIDDEN VARIABLE model can reproduce the quantum predicCons.
o “NO MODEL WHERE PHOTON KNEW IN ADVANCE CAN ACCOUNT FOR THE OBSERVED STATISTICS”
A Quantum Delayed-‐Choice (QDC) Experiment
Classical delayed-‐choice Equivalent quantum network
Delayed-‐choice with quantum beam spliber Biased QRNG
A Quantum Delayed-‐Choice (QDC) Experiment
Classical delayed-‐choice
Equivalent quantum network
o Second beam spliper inserted/removed randomly once the photon is inside the MZI (has passed BS1)
o HADAMARD gates act as BEAMSPLITTERS o RED LINE = ANCILLA, iniCally prepared in |0>, then
|+>, then measured – acts as QRNG!
A Quantum Delayed-‐Choice (QDC) Experiment
Delayed-‐choice with quantum beam spliber Equivalent quantum network
[Classical control acer ANCILLA measurement] = [quantum control before ANCILLA measurement SECOND BEAM SPLITTER NOW IN SUPERPOSITION OF PRESENT / ABSENT (controlled-‐Hadamard) C(H)
o Have a quantum BS in superposiCon of present / absent o Interferometer is therefore open / closed o Forces photon to be in a superposiCon of parCcle / wave (following Wheelers interpretaCon)
A Quantum Delayed-‐Choice (QDC) Experiment
Delayed-‐choice with quantum beam spliber
Biased QRNG
o Can also bias the QRNG by preparing the ANCILLA in an arbitrary state
A Quantum Delayed-‐Choice (QDC) Experiment
QUANTUM DELAYED CHOICE
o BS2 in now a quantum BS (represented by a controlled-‐Hadamard operaCon) – can be set into a
superposiCon of present and absent via ANCILLA photon o Again, a single photon is sent through the MZI, aher the first Hadamard a superposiCon of the two
spaCal modes is created (|+>) o And a phase shiher modifies the relaCve phase
A Quantum Delayed-‐Choice (QDC) Experiment
THE PRESENCE OF BS2 DEPENDS ON STATE
OF ANCILLA o If ancilla is prepared in the state |0>a no
BS is present
o MZI is leh OPEN
o Corresponds to IDENTITY acCng upon the state, results in:
o Final measurement in the computaConal {|0>s,|1>s} basis reveals the PARTICLE-‐NATURE of the photon as a WHICH-‐PATH measurement is made.
o Measured intensiCes in both output modes are EQUAL and PHASE-‐INDEPENDENT
ID’ = ID’’ = 1/2
A Quantum Delayed-‐Choice (QDC) Experiment
THE PRESENCE OF BS2 DEPENDS ON STATE
OF ANCILLA
o If ancilla is prepared in the state |1>a the
BS is present
o MZI is CLOSED
o Corresponds to HADAMARD acCng upon the state, results in:
o Final measurement gives informaCon regarding the MZI phase, but no path informaCon
o Measured intensiCes are now
A Quantum Delayed-‐Choice (QDC) Experiment
o Main feature of QBS is that it can be in a superposiCon of present and absent
o If ANCILLA is iniCally in a superposiCon the global state of the system evolves into:
o The SYSTEM and ANCILLA photons are now ENTANGLED when
o Measured intensiCes are now
A Quantum Delayed-‐Choice (QDC) Experiment
A Quantum Delayed-‐Choice (QDC) Experiment -‐ IMPLEMENTATION
A Quantum Delayed-‐Choice (QDC) Experiment – (CHSH) BELL INEQUALITY
A Quantum Delayed-‐Choice (QDC) Experiment – (CHSH) BELL INEQUALITY
A Quantum Delayed-‐Choice (QDC) Experiment
A Quantum Delayed-‐Choice (QDC) Experiment